With malnutrition and cholera major concerns, the UN says it urgently needs more workers in place in northern Rakhine state.
Hundreds of thousands more Rohingya could flee Myanmar, the UN has said, as it warned of the risk of a “further exodus”.
The UN also slammed the level of access granted by Myanmar to the heart of the refugee crisis as “unacceptable”.
Around 515,000 people have crossed into Bangladesh in the last six weeks after a military crackdown that the UN has described as ethnic cleansing.
Refugees are living in camps near the border amid claims that hundreds of villages were torched in an apparent attempt to force Muslims out of the Buddhist-majority country.
A small UN team visited the area this week and said the suffering was “unimaginable”.
Mark Lowcock, head of the UN’s humanitarian office, said: “This flow out of Myanmar has not stopped yet, it’s into the hundreds of thousands of Rohingya (who are) still in Myanmar, we want to be ready in case there is a further exodus.”
“Half a million people do not pick up sticks and flee their country on a whim.”
An estimated 2,000 refugees are still arriving in Bangladesh every day, according to the International Organisation for Migration.
Charities have warned of a food crisis across the border and say 281,000 people desperately need food, including 145,000 children under five.
The World Health Organisation also says cholera is also a major risk.
Some 900,000 doses of vaccine are due to arrive in the region at the weekend.
Mr Lowcock said the access given to humanitarian workers in northern Rakhine state, where the crisis is centred, is “unacceptable”.
Access to much of the area has been blocked by Myanmar authorities.
But Mr Lowcock said he believed a “high-level” team would be able to visit the area in the next few days.
The Rohingya are regarded as illegal immigrants in Myanmar and most are stateless, but authorities reject any suggestion of ethnic cleansing.
The country’s army also claims that Rohingya militants have set fire to houses in recent days to force more people towards the border.
Aung San Suu Kyi, Myanmar’s leader, was criticised for not doing moreto intervene – but has since spoken out against any human rights abuses.
She has said Myanmar will take back anyone verified as a refugee, but that could take months or years and is unlikely to solve the immediate crisis.
With the influx of people showing no signs of stopping, Bangladesh said on Thursday it would build one of the world’s biggest refugee camps.
They plan to expand an existing camp near the border town of Cox’s Bazar so that it can accommodate around 800,000 people.